The present invention relates to a waveform display apparatus of the type in which analog waveform data is converted to digital data, and then temporarily stored in a memory. The converted analog waveform is read out from the memory and displayed on a raster scanning type display unit.
A waveform display apparatus formed by the combined use of a memory and a raster scanning type display unit is excellent in that when analog data is temporarily stored in a memory, the data can be reproduced repeatedly. Further, since it employs the raster scanning type display unit, it has the advantage that a character or the like can also be displayed along with the waveform.
This type of waveform display apparatus requires a memory which has the capacity to store all one frame picture elements. Therefore, in order to enhance the resolution of a waveform display, it is necessary to use a memory of increased storage capacity. Even when such a memory is employed, the number of memory elements actually used for storing an analog waveform is so small that there is a very low efficiency in the utilization of the memory.
Japanese Patent Application No. 51-9114, Laid Open Feb. 16, 1978 discloses a graphic display device designed to display an analog waveform using a small storage capacity memory. This graphic display device employs a memory which has the same number of addresses as the number of picture elements into which the display surface of a display unit is divided in the main scanning direction of raster scanning. A-D converted analog data values to be displayed are sequentially stored in the memory in address order; the memory is read out in synchronism with each picture element for each raster scan; the data thus read out is compared with the number of the raster; and upon each coincidence between them, a dot is displayed.
The known graphic display device described above has a resolution corresponding to the number of picture elements in the main scannng direction of raster scanning (for example, in the X-axis direction) and a resolution corresponding to the number of scanning lines in the direction perpendicular to the main scanning direction (i.e. in the Y-axis direction). Consequently, an analog waveform which varies relatively gently can be displayed as a continuous analog waveform. But in the case of displaying an analog waveform which varies steeply and greatly in the Y-axis direction, adjacent dots displaying the waveform are greatly spaced apart, making the display difficult to analyze.
To facilitate a better understanding of the present invention, a description is presented, with reference to FIGS. 1 to 3, of the conventional waveform storage apparatus disclosed in the Japanese Patent Application No. 51-9114, Laid Open Feb. 16, 1978.
Controller 101 writes into data memory 102, data 100 provided from an external computer or the like. Data memory 102 has the same number of addresses as picture elements, aligned in the direction of raster on the display surface of display unit 107 employing a raster scanning type, for example, electromagnetic deflection type cathode ray tube. The data stored in data memory 102 is sequentially read out by a signal from timing controller 106 for each raster scan.
A numerical signal DQ corresponding to the data read out of data memory 102, is provided to comparator 103 and a color specifying signal CL is applied to color video signal generator 104. Comparator 103 compares the numerical signal DQ with a raster scanning line number Y supplied from timing controller 106 and yields, for example, a high, "H" or low, "L" logic signal DS depending respectively on whether or not the numbers are equal.
Color video signal generator 104 produces a color video signal based on the color specifying signal CL read out of data memory 102, the output signal DS from comparator 103 and a timing signal CP from the timing controller 106. The color video signal is applied to synchronous brightness controller 105. The synchronous brightness controller 105 controls, in accordance with the color video signal and a timing signal, display unit 107 to provide a video display.
The operation of the apparatus is as follows. Timing controller 106 supplies synchronous brightness controller 105 with a vertical synchronizing signal V and a horizontal synchronizing signal H depicted in FIGS. 2A and 2B, respectively. Further, timing controller 106 supplies comparator 103 with a raster number shown in FIG. 2C, and supplies data memory 102 and color video signal generator 104 with a clock signal CP as depicted in FIG. 2D.
On the display surface of display unit 107 there is provided an effective display area for producing an analog display. The effective display area is defined by two rasters Ho and Hm in the Y-axis direction and by two picture-element positions, that is, clock pulses CPo and CPm in the X-axis direction (a raster-scanning direction). Rasters in the effective display area are given raster numbers m, m-1, m-2, . . . in a sequential order.
In FIG. 2C, the raster numbers m, m-1, m-2, . . . are shown to vary in a stairstep manner in analog form but, in practice, they are provided as digital signals. During the scanning of one raster, the same number of clock pulses CP are generated as there are picture elements in the direction of the raster and, data memory 102 is read out by those clock pulses lying in the effective display area. The data read out from addresses S.sub.1 to S.sub.n of data memory 102 is compared by comparator 103 with the raster number, and when the address and raster number are equal, comparator 102 produces the "H" logic level output. The "H" logic level output is converted by color video signal generator 104 to a spot signal of a specified color. The spot signal is applied via synchronous brightness controller 105 to display unit 107. Therefore, for each application of the spot signal to display unit 107, a spot is displayed on the display surface at one of the positions corresponding to the addresses S.sub.1 to S.sub.n of data memory 102, as depicted in FIG. 3. For example, if the data stored in data memory 102 at addresses S.sub.10 and S.sub.20 is Y.sub.1 as shown in FIG. 3, then the data stored at each of these addresses are detected to be equal at the time of scanning the raster having a raster number Y.sub.1, and a spot display is produced.
The raster scanning takes place sequentially, changing the raster number from the upper limit to the lower limit of effective display area. Accordingly, all data having a value in the effective display area is displayed in the form of spots; that is, an analog waveform is displayed by the spots.
With the above-described apparatus, it is sufficient that data memory 102 has the same number of addresses as the picture elements in the direction of the raster. Accordingly, the capacity of data memory 102 is a few hundredths of the capacity of a conventional refresh memory which is required to be equal to the total number of picture elements of one picture frame. Further, the data to be stored in data memory 102 may be in the form of digital signals, that is, they need not be converted to picture signals, so that no pattern generator is required. Therefore, the storage system is greatly simplified.
When this conventional waveform display is utilized in a spectrum analyzer, it displays the frequency spectra of electric waves or frequency characteristics of circuit elements. As a result, the spectrum analyzer often displays waveforms such as depicted in FIG. 4A, which have abrupt changes in the Y-axis direction. When waveforms 401, 402 and 403 shown in FIG. 4A are displayed using the apparatus described above, the adjacent dots or picture elements are greatly spaced in the Y-axis direction as shown in FIG. 4B. This makes recognition of the display difficult. This is especially true for waveforms such as 402, which rise and fall very sharply with narrow pulse width. This results in displays such as shown in FIG. 4B, where only one dot, 405, in the vicinity of the waveform peak is displayed. Display of such a single, small dot 405 is likely to be overlooked, introducing the possibility of serious measurement errors. Additionally, when waveforms with small rise times, such as those shown in FIG. 4A, 401, 402 and 403, are sampled at equal time intervals, the peak points of the waveforms are not always sampled. Therefore, even if the waveforms are displayed based on the data read out from data memory 102 as depicted in FIG. 4B, the uppermost dots 404, 405 and 406 of the respective waveforms do not necessarily indicate the peak values of the corresponding analog waveform. Accordingly, the prior art apparatus has the defect that the magnitude of the spectrum may not be accurately reproduced.